THE O-TYPE BINARY 15-MONOCEROTIS NEARS PERIASTRON

We present new radial velocity measurements for the massive binary 15 Monocerotis which indicate that the system is now very close to perias tron (1996.9) in its 24 yr orbit. The velocity separation in the comin g year may be large enough to permit an accurate estimate of mass rati o. We also present our first astrometric measurement of 15 Mon made wi th the Hubble Space Telescope Fine Guidance Sensors (FGS). The FGS tra nsfer functions are consistent with an advanced orbital position close to periastron, and we present preliminary orbital elements for the co mbined spectroscopic and astrometric orbit.